12v Battery Is Discharging, Re-Charge Now !

Here is my reading of 12v battery on today's ~50 miles drive. I run a few errands along with my weekly work-related drive.

1) 2020-04-19 20:23 13.06v. 12v fully charged. No opening of door or fob near the car during this time.
2) 2020-04-25 11:37 12.58v From 13.06v right after fully charged the battery drained down to 12.58v in 6 days.
3) 2020-04-25 11:55 12.24v In preparation for the trip. I had to keep the hatch door open for ~17min. After opening the driver side door the 12v reading dropped as low as 12.24v before starting the car.
4) 2020-04-25 11:55-12:58 14.42v During 32 miles trip (1 hour) the 12v was being charged at constant ~14v. The charging started as soon as I started the car.
5) 12:58-14:11 12.69v low at PARK. I had two additional stops I made and drove ~18 miles. When car was parked 12v dropped to as low as 12.69v, but as soon as the car is started it gets charged at ~14v, but after a while, the charge voltage dropped to ~13v for floating charge most of the way back home.
6) 14:13 12.69v After reaching home and turning off the car and getting out of the car, 12v reading dropped to the 99% 12.69v.
7) 14:31-16:17 12.81v-12.78v After the car was turned off, in the next 18 min the battery charge came up and stabilized at ~12.8v 100%.

It looks like the brand new 12v battery in PRIME after fully charged keeps the SOC above 90% 12.56v during 6 days PARK without disturbance. And ~1 hour of driving is sufficient to bring it back to a full charge again. My current schedule should be enough to keep the battery in good shape, as long as I do not open the door and use 12v during PARK.

I will plug in L1 EVSE later to top off the traction battery to SOC level 35-45%. I will check what happens to 12v during hybrid battery plug-in, but I don't think it will add much charge to already 100% 12v battery.

 

so after a week, has anyone been able to let the car sit and get 7 readings on the battery?

 

7 readings?

 

one each day since 04-18?

 

so that difference is the surface charge?

 

I started my test on 04-19 (Sun).

4-19 12.56v before charging 13.06v after fully charged
4-20 12.87v
4-21 12.74v
4-22 12.67v
4-23 12.61v
4-24 12.57v
4-25 12.56v drove ~50 miles, aux batt back to 12.75v

2 hours after the car was parked, the AUX battery was still 12.75v. I plugged in L1 to my car for ~2.5 hours 2.88khw up to SOC43%. When L1 is plugged in, the AUX bat reading went up to 13.31v immediately and the AUX bat was charged briefly. But it stopped very quickly, less than a minute. The AUX battery stayed ~12.7v during plug-in and after. I am going to let the car sit another week.

 

i suppose a battery isn't linear? is that why the drop gets smaller each day?

 

Yeah, that was the case with mine. Mine did stabilize at around 12.56v.

 

Or it's currently somewhat stable around 12.56? Continuing to have protracted downtime without charging, I'd speculate that stable point will slowly drop.

 

Isn't it the natural degradation of capacity for any battery? My almost brand new 12v battery seems to have stabilizing capacity at 12.56v, but Rob's 3 years old battery has much lower capacity at resting state due to natural degradation?

 

Differences in temperatures could also be a factor in the amount of the drop right? Or was the temperature pretty much the same each day? But anyway I suspect the discharge rate wouldn't be 100% linear, even for a resting battery under laboratory conditions!

 

Yes, my 24-hour monitoring of 12v battery reveals the lowest SOC is at early morning day low temp and then SOC actually goes up a bit during the day and highest at day high in the afternoon.

According to @Trollbait the amount of self-discharge is higher at higher temperatures. I believe that is due to the battery chemistry. But in my climate around this time of year, daytime high of 45 and low of 25 seems to favor the higher temp for more SOC in the 12v. But YMMV.

As for the effect of the temperature on the battery life, it is known that the higher ambient temperature shortens the battery lifespan. In my region, average 12v battery life is estimated to be 5 years, but I have never had any 12v battery fail in less than 6 years in our cars in the past. I had several OEM batteries that lasted over 10 years before dying. This without any precautionary maintenance.

 

The demarcations on that map should maybe dip in the middle, climb at the two coasts? More so on the west coast, since the eastern seaboard seems to experience tougher winters? Anyhoo...

 

Yeah, that was the first image on Google search. Is this one better?

Batteries Etc. - Home

 

Mmmm, okay.

 

It will be interesting to see if it starts at under 12v.